For economic reasons, almost all large electric utilities are interconnected with other utilities through tie lines. This arrangement allows the interconnected utilities to take advantage of the time difference at which peak loads occur in several systems and to decrease the amount of spinning reserve they must have in order to ensure a continuous supply of electricity for their customers even when they experience a serious loss of generating capacity. Thus, the large power grids in all developed nations in the world are characterized by very large power generation canters interconnected by tie lines to other generation and load canters. The amount of steady state power that can be transferred over a tie line is determined to a large extent by the requirement to maintain stability under fault conditions. When a fault occurs in or near a large power center, the generation in the center tends to accelerate and if the resulting angular swing between the faulted center and other canters is large enough, some tie lines may have to be disconnected.
One method of increasing the stability of the system or, what is equally important, of increasing the allowable power transfer along the tie line, is to use dynamic braking resistors to inhibit the acceleration of the generating system under fault conditions. Braking resistors consist of large three phase resistor banks which are connected as loads to the power system when measurements indicate that tie line synchronization may be lost. The resistor bank is injected into the system for a very short time only (typically about one second) and then is automatically disconnected again. The resistor bank has a very short time rating but in the period of approximately one second is able to absorb megajoules or even gigajoules of energy thus limiting the acceleration of the generating capacity in the system and thereby preventing loss of synchronization.
Dynamic braking resistors have been installed in a number of places around the world and have proven their worth. The biggest factor inhibiting their much broader use is the cost of the resistor installations. The resistors described in this disclosure will reduce the cost of installations considerably.